This invention relates to superconductors in general and more particularly to a superconductor with improved stabilization.
Superconducting composite conductors consisting of several strands with a superconductive intermetallic compound of at least two elements, and at least one strand of a thermally and electrically highly conductive stabilizing metal which is normally conducting at the operating temperature of the superconducting composite conductor, wherein the strands with the superconductive compound each contain a core of at least one higher melting point element of the compound, having, at least on its surface, a layer of the compound, embedded in an alloy of at least one lower melting point element of the compound and a carrier metal, in the form of a cable, stranded wire or flat cable, are known from German Auslegeschrift No. 23 45 779, especially column 6, line 16, to column 8, line 51, and German Offenlegungsschrift No. 26 54 924, especially page 30, para. 1, and page 64, last para., to page 67, para. 2.
Starting out from an intermediate product which consists of an alloy of a carrier metal and at least one lower melting point element of the superconductive compound and one or more cores embedded in the alloy of at least one higher melting point element of the compound, the superconductive intermetallic compound is formed in such conductors by a heat treatment, in which the lower melting point element of the compound diffuses into the core of the higher melting point element and reacts with the core material, forming the compound. Depending on the composition of the alloy, the dimensions of the intermediate product and the duration of the heat treatment, a surface layer of the core or also the entire core, can be converted into the superconductive compound.
In practice, the superconductive intermetallic compounds Nb.sub.3 Sn and V.sub.3 Ga, in particular, are used at present, both of which have A-15 crystal structure. Both compounds have very good superconducting properties and are distinguished particularly by a high transition temperature, a high critical magnetic field and high critical current density. In order to manufacture superconductors with these compounds, one starts out, as a rule, with an intermediate product consisting of a matrix of a copper-tin alloy or of a copper-gallium alloy in which a multiplicity of niobium or vanadium cores is embedded. This intermediate product is first processed to reduce the cross section, drawing the cores into thin filaments. Subsequently, the heat treatment for forming the compound takes place. In addition to the two compounds mentioned, however, other compounds of two or more components with the same crystal structure, such as Nb.sub.3 Ga, Nb.sub.3 Al, V.sub.3 Ga, V.sub.3 Si or Nb.sub.3 (Al.sub.0.8 Ge.sub.0.2) as well as intermetallic superconductive compounds with other cyrstal structures are also of interest.
Certain difficulties arise in superconductors with superconductive intermetallic compounds due to the fact that the superconductive compounds are relatively brittle. The flexibility of the finished conductors is therefore lower, as a rule, than that of comparable conductors which contain cores of superconductive alloys such as niobium-titanium. One therefore attempts to make the layers of the superconductive compounds on the surface of the core and, also, the cores themselves as thin as possible. Since heavy cross section reductions of the intermediate products are required for this purpose, the conductor strands containing the cores themselves also have a relatively small cross section as a rule. This has the advantage that the cores within a conductor strand are located relatively close to the neutral axis when the conductor is bent, for instance, in winding a coil, so that the mechanical tensile and compression stresses occurring in the compound layers can be kept within limits even for relatively small bending radii. However, if large currents are to be obtained, a number of thin individual strands must be combined in a conductor of larger cross section in which the superconducting layers of the individual conductor strands are again farther removed from the neutral axis.
In superconductors with intermetallic compounds, the electrical stabilization of the superconductors is a further problem. Stabilization requires a metal of high thermal and electric conductivity which is electrically normally conducting at the operating temperature of the superconductor. In contrast to superconductors, in which, for instance, thin filamentary cores of niobium-titanium are embedded in a copper matrix, the alloy material surrounding the cores with an intermetallic compound can be utilized for stabilization only with difficulty. For, since, even after the superconductive compound is formed, the alloy material still contains residues of the lower melting point element or several such elements of the compound, it has a substantially higher electrical resistance than, for instance, pure copper, at the operating temperature of the superconductor, which is below the critical temperature of the respective superconductor material, i.e., as a rule between about 1 and 20 K. To achieve better stabilization, strands of stabilizing metal, for instance, copper, are provided in the superconducting composite conductors known from Auslegeschrift No. 23 45 779 and Offenlegungsschrift No. 26 54 924, in addition to the strands with the superconductive compound. So as to also achieve maximum flexibility of the conductor after the heat treatment for forming the superconductive compound, however, separator means which prevent adjacent strands from sticking together, especially due to diffusion during the heat treatment, are arranged in these known conductors between the individual strands. As a consequence, no intimate electrical and thermal contact is formed between the strands with the superconductive compound and the strands of stabilizing metal. This, in turn, can have a very unfavorable effect on the stabilizing action of the strands of stabilizing metal. In the known composite conductors, therefore, in order to provide a further improvement of the stabilization, the possibility of arranging additional zones of stabilizing metal, within the individual strands with the superconductive compound, which zones extend along the strand and are enclosed by a diffusion-inhibiting layer, for instance, of niobium, vanadium or tantalum, is disclosed. This diffusion inhibiting layer acts to prevent diffusion of the lower melting point element of the compound into the stabilizing metal during the heat treatment, and, thereby, prevents an increase of the electric resistance of the stabilizing metal (see, for instance, German Auslegeschrift No. 23 45 779, col. 8, line 55, to col. 10, line 40, and German Offenlegungsschrift No. 26 54 924, page 28, para. 3, to page 29, last line).
Such stabilizing zones within the conductor strands, however, have the disadvantage, for one, that they increase the cross section of the strand, whereby the distance of the cores from the neutral axis of the conductor strand is increased, at least if the stabilizing zone is arranged in the center. This in turn also increases the distance of the layers of the superconducting intermetallic compound from the neutral axis. Secondly, the fabrication of such conductor strands is accompanied by great problems due to the different material properties of the cores, alloy jacket, diffusion inhibitor and stabilizing metal. In particular, cracks can readily occur in the diffusion inhibiting layer, through which cracks the lower melting point element of the compound can penetrate into the stabilizing metal. In such a case, the entire conductor strand, including the superconductor material, then becomes unusable.